Known in the art is a process for producing zinc phosphate by way of treating zinc sulphate with a solution of phosphoric acid in the presence of an alkali (cf. French Pat. No. 812,002).
This prior art process has a disadvantage residing in that along with the formation of one ton of zinc phosphate there is formed one ton of sodium sulphate which is a waste product and finds no commercial application. Furthermore, paint coatings including zinc phosphate produced by this process have insufficient protective properties. Thus, the surface of a paint coating based on epoxy resin containing said zinc phosphate is damaged as much as 50% by blistering after residence of the coating for 6 months in a 3% aqueous solution of sodium chloride at a temperature of 20.degree. C. Physico-mechanical properties of the coating are also insufficient (thus, the impact strength of the coating is equal to 15 kgf.cm).
Also known in the art is a process for producing zinc phosphate by treating zinc oxide with a solution of phosphoric acid at the boiling point of the solution (121.degree. C.), followed by cooling of the solution and separation of a crystalline product (cf. G. Brower "Guidebook on Preparative Inorganic Chemistry", 1956, p. 497).
The yield of zinc phosphate in this process is low and constitutes about 20%. Furthermore, paint-varnish coatings incorporating zinc phosphate produced by the above-mentioned process have insufficient protective properties. Thus, the surface of a paint coating based on epoxy resin containing said zinc phosphate after exposure of the coating for 6 months to a 3% aqueous solution of sodium chloride at a temperature of 20.degree. C. is damaged by 20% with blistering corrosion. Moreover, the coatings have low physico-mechanical properties. (Thus, impact strength of the coating is 20-25 kgf/cm).